As known, for operator-assisted call accounting, it is necessary to collect and correlate data from different connecting points, for example from switchboards and some other points of an exchange: more particularly, it is worth knowing the identity of the switchboard and semicord used for the call (datum present at the switchboard) and the identity of a trunk engaged by each semicord (datum present at the exchange).
Devices for detecting and correlating information coming from different points of a connection, particularly the identity of a calling subscriber and the identity of a called subscriber, are known in the art of telephone switching. Any stage marker of an indirect-control exchange represents an example.
Yet these devices detect the two ends of a connection to be set up and set up the connection by choosing a free path. Then, they cannot be used when, as for the operator-assisted call accounting, it is necessary to know the data of a connection set up manually by an operator without the help of automatic devices.
Furthermore, because the conventional identifiers operate on automatic exchanges, they cannot operate when the same line is used by many connections at the same time, i.e. in the case of a conference among many operators or of a connection transfer from an operator to another.